The present invention generally relates to the field of image reconstruction in computed tomography (CT) systems and more particularly to a method and apparatus for reducing artifacts in image data generated by computed tomography systems.
CT scanners operate by projecting fan-shaped or cone-shaped X-ray beams from an X-ray source. The X-ray source emits X-rays at numerous angular positions relative to an object being imaged, such as a patient, which attenuates the X-ray beams as they pass through the object or patient. The attenuated beams are detected by a set of detector elements, which produce signals representing the intensity of the incident X-ray beams. The signals are processed to produce data representing the line integrals of the attenuation coefficients of the object along the X-ray paths connecting the X-ray source to the detector elements. These signals are typically called “projection data” or just “projections”. The individual attenuation values in a given projection are typically referred to as data points. By using reconstruction techniques, such as filtered backprojection, useful images may be formulated from the projections. The images may in turn be associated to form a volume rendering of a region of interest. In a medical context, pathologies, specific physiological regions, or other structures of interest may then be located or identified from the reconstructed images or rendered volume. In non-medical contexts, otherwise inaccessible items or structures, such as items within a bag or package, may be observed and/or studied.
Reconstructed images in CT systems often exhibit a blurring of structures in the direction of the radiation beams that were used to acquire the projection data. Volumetric images obtained with most reconstruction methods exhibit artifacts; such artifacts can be due to high-contrast structures in the imaged volume. These artifacts make image interpretation and visualization steps difficult. The artifacts associated with an imaged structure vary depending on the orientation of the structure with respect to the acquisition geometry and the opacity of different features within the structure, such as long straight edges or areas of high X-ray attenuation. Therefore, the blurring of structures, or edges between structures, may create undesirable image artifacts and inhibit the separation of structures in the reconstruction of the imaged volume.
Different filtering techniques are used during reconstruction of the volumetric images to try to minimize or eliminate artifacts and enhance area definitions that represent features of interest in the three-dimensional structure. A filter is selected for processing the projection data prior to the backprojection. Depending on the filter chosen, either the spatial resolution or contrast sensitivity can be enhanced, at the expense of degrading the other. Contrast sensitivity is a measure of the ability to detect a region in a reconstructed image with a density value that is slightly different from the surrounding background in the presence of noise. For example, an increase in spatial resolution through filtering will result in a decrease in contrast sensitivity, and vice-versa. A further limitation of this approach is the selection of a single filter for the backprojection of all projection data, without regard for changes in features or surfaces throughout the imaged volume. Typically a filter, referred to herein as a standard filter, is used which represents a balance between spatial resolution and contrast sensitivity. An improved filtering technique for dealing with subtle contrast changes between features, i.e. improved contrast sensitivity, without compromising spatial resolution is in certain cases desirable. Alternatively, an improved filtering technique for increasing the spatial resolution for selected regions, without compromising contrast sensitivity, may be desirable.